The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process fluid temperature transmitter provides an output related to a sensed process substance temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled. In order to monitor a process fluid temperature, the transmitter includes a temperature sensor, such as a thermocouple.
A thermocouple is fabricated by joining two dissimilar metals, such as bismuth and antimony. The junction of the two dissimilar metals produces a small voltage that is related to its temperature. This is known as the Seebeck effect. Process fluid temperature transmitters that employ thermocouple sensors, thus measure the small voltage of the thermocouple, and then calculate process fluid temperature based upon the thermocouple voltage. Although a thermocouple""s primary variable of interest is its voltage (indicative of temperature) it is generally known that the thermocouple""s resistance is indicative of its condition. As thermocouples age, or otherwise degrade, thermocouple resistance changes. Thus, thermocouple resistance measurement can be used to evaluate the condition of the thermocouple. In order to measure the resistance, a test current is generally passed through the thermocouple, and the resulting voltage is measured and used to calculate the resistance.
In two-wire process control installations, process measurement devices, such as temperature transmitters can receive all required electrical power through the same two wires that are used for data communication. Generally, the amount of power available on the loop is limited in order to facilitate compliance with intrinsic safety requirements. Typically, the loop current varies between 4 and 20 mA to indicate a process variable. Thus, a device powered by the loop must be operable on 4 mA or less. Such minimal electrical power generally limits the computational capacity of a given process device, as well as the amount of power that can be used for diagnostics. Thus, there is a tradeoff between the convenience of two-wire temperature transmitters, and the ability to provide suitable amounts of diagnostic current through a thermocouple to achieve accurate diagnostic information.
As process control becomes more accurate, there is an increasing need to provide process devices that not only provide process variables, but also indicate their own health. By providing enhanced process device diagnostics, process variable information can be relied upon to a greater or lesser extent, depending upon the state of the process device. Providing such devices will enhance process control and potentially increase the efficiency of predictive maintenance.
A two-wire temperature transmitter performs thermocouple diagnostics on a thermocouple attached to the transmitter to determine if, and the extent to which, the thermocouple has degraded. The transmitter passes a diagnostic current through a thermocouple to obtain the resistance of the thermocouple. The resistance is then used to calculate a diagnostic output that is related to thermocouple degradation. Various methods of obtaining thermocouple resistance are also provided.